Method SynthTextPaneUI.installUI(JComponent c) says:

 "... Sets the associated component to opaque
  if the opaque property has not already been set
  by the client program. ..."

Actually it doesn't happen.
Please see the following code sample:

---
import javax.swing.*;
import javax.swing.plaf.synth.*;

public class InstallUIComponentOpaque {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override public void run() {
                try {
                    UIManager.setLookAndFeel(new SynthLookAndFeel());
                } catch (UnsupportedLookAndFeelException e) {
                    throw new RuntimeException(e);
                }
                new JTextPane().setUI(new SynthTextPaneUI() {
                    @Override public void installUI(JComponent c) {
                        System.out.println("c.isOpaque() = " + c.isOpaque());
                        super.installUI(c);
                        System.out.println("InstallUIComponentOpaque.installUI called");
                        System.out.println("c.isOpaque() = " + c.isOpaque());
                    }
                });
            }
        });
    }
}
---

For JDK7 b93 output will be:

c.isOpaque() = false
InstallUIComponentOpaque.installUI called
c.isOpaque() = false
For JDK7 b98 the output is still

c.isOpaque() = false
InstallUIComponentOpaque.installUI called
c.isOpaque() = false